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588df31a | 1 | .\" -*- nroff -*- |
2 | .\" | |
3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> | |
4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland | |
5 | .\" All rights reserved | |
6 | .\" | |
7 | .\" As far as I am concerned, the code I have written for this software | |
8 | .\" can be used freely for any purpose. Any derived versions of this | |
9 | .\" software must be clearly marked as such, and if the derived work is | |
10 | .\" incompatible with the protocol description in the RFC file, it must be | |
11 | .\" called by a name other than "ssh" or "Secure Shell". | |
12 | .\" | |
13 | .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. | |
14 | .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. | |
15 | .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. | |
16 | .\" | |
17 | .\" Redistribution and use in source and binary forms, with or without | |
18 | .\" modification, are permitted provided that the following conditions | |
19 | .\" are met: | |
20 | .\" 1. Redistributions of source code must retain the above copyright | |
21 | .\" notice, this list of conditions and the following disclaimer. | |
22 | .\" 2. Redistributions in binary form must reproduce the above copyright | |
23 | .\" notice, this list of conditions and the following disclaimer in the | |
24 | .\" documentation and/or other materials provided with the distribution. | |
25 | .\" | |
26 | .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR | |
27 | .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES | |
28 | .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. | |
29 | .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, | |
30 | .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT | |
31 | .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, | |
32 | .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY | |
33 | .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT | |
34 | .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF | |
35 | .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. | |
36 | .\" | |
ea46e550 | 37 | .\" $OpenBSD: sshd_config.5,v 1.68 2006/07/21 12:43:36 dtucker Exp $ |
588df31a | 38 | .Dd September 25, 1999 |
39 | .Dt SSHD_CONFIG 5 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd_config | |
43 | .Nd OpenSSH SSH daemon configuration file | |
44 | .Sh SYNOPSIS | |
45 | .Bl -tag -width Ds -compact | |
46 | .It Pa /etc/ssh/sshd_config | |
47 | .El | |
48 | .Sh DESCRIPTION | |
32cfd177 | 49 | .Xr sshd 8 |
588df31a | 50 | reads configuration data from |
51 | .Pa /etc/ssh/sshd_config | |
52 | (or the file specified with | |
53 | .Fl f | |
54 | on the command line). | |
55 | The file contains keyword-argument pairs, one per line. | |
56 | Lines starting with | |
57 | .Ql # | |
58 | and empty lines are interpreted as comments. | |
533b9133 | 59 | Arguments may optionally be enclosed in double quotes |
60 | .Pq \&" | |
61 | in order to represent arguments containing spaces. | |
588df31a | 62 | .Pp |
63 | The possible | |
64 | keywords and their meanings are as follows (note that | |
65 | keywords are case-insensitive and arguments are case-sensitive): | |
66 | .Bl -tag -width Ds | |
61a2c1da | 67 | .It Cm AcceptEnv |
68 | Specifies what environment variables sent by the client will be copied into | |
69 | the session's | |
70 | .Xr environ 7 . | |
71 | See | |
72 | .Cm SendEnv | |
73 | in | |
74 | .Xr ssh_config 5 | |
75 | for how to configure the client. | |
b8b9f2e6 | 76 | Note that environment passing is only supported for protocol 2. |
61a2c1da | 77 | Variables are specified by name, which may contain the wildcard characters |
f09ffbdb | 78 | .Ql * |
61a2c1da | 79 | and |
80 | .Ql \&? . | |
b8b9f2e6 | 81 | Multiple environment variables may be separated by whitespace or spread |
61a2c1da | 82 | across multiple |
83 | .Cm AcceptEnv | |
84 | directives. | |
b8b9f2e6 | 85 | Be warned that some environment variables could be used to bypass restricted |
61a2c1da | 86 | user environments. |
87 | For this reason, care should be taken in the use of this directive. | |
88 | The default is not to accept any environment variables. | |
31b41ceb | 89 | .It Cm AddressFamily |
90 | Specifies which address family should be used by | |
32cfd177 | 91 | .Xr sshd 8 . |
31b41ceb | 92 | Valid arguments are |
93 | .Dq any , | |
94 | .Dq inet | |
b74c3b8c | 95 | (use IPv4 only), or |
31b41ceb | 96 | .Dq inet6 |
97 | (use IPv6 only). | |
98 | The default is | |
99 | .Dq any . | |
588df31a | 100 | .It Cm AllowGroups |
101 | This keyword can be followed by a list of group name patterns, separated | |
102 | by spaces. | |
103 | If specified, login is allowed only for users whose primary | |
104 | group or supplementary group list matches one of the patterns. | |
588df31a | 105 | Only group names are valid; a numerical group ID is not recognized. |
106 | By default, login is allowed for all groups. | |
84c1b530 | 107 | The allow/deny directives are processed in the following order: |
108 | .Cm DenyUsers , | |
109 | .Cm AllowUsers , | |
110 | .Cm DenyGroups , | |
111 | and finally | |
112 | .Cm AllowGroups . | |
ac1ec4d8 | 113 | .Pp |
114 | See | |
115 | .Sx PATTERNS | |
116 | in | |
117 | .Xr ssh_config 5 | |
118 | for more information on patterns. | |
588df31a | 119 | .It Cm AllowTcpForwarding |
120 | Specifies whether TCP forwarding is permitted. | |
121 | The default is | |
122 | .Dq yes . | |
123 | Note that disabling TCP forwarding does not improve security unless | |
124 | users are also denied shell access, as they can always install their | |
125 | own forwarders. | |
588df31a | 126 | .It Cm AllowUsers |
127 | This keyword can be followed by a list of user name patterns, separated | |
128 | by spaces. | |
1be697b6 | 129 | If specified, login is allowed only for user names that |
588df31a | 130 | match one of the patterns. |
588df31a | 131 | Only user names are valid; a numerical user ID is not recognized. |
132 | By default, login is allowed for all users. | |
133 | If the pattern takes the form USER@HOST then USER and HOST | |
134 | are separately checked, restricting logins to particular | |
135 | users from particular hosts. | |
84c1b530 | 136 | The allow/deny directives are processed in the following order: |
137 | .Cm DenyUsers , | |
138 | .Cm AllowUsers , | |
139 | .Cm DenyGroups , | |
140 | and finally | |
141 | .Cm AllowGroups . | |
ac1ec4d8 | 142 | .Pp |
143 | See | |
144 | .Sx PATTERNS | |
145 | in | |
146 | .Xr ssh_config 5 | |
147 | for more information on patterns. | |
588df31a | 148 | .It Cm AuthorizedKeysFile |
149 | Specifies the file that contains the public keys that can be used | |
150 | for user authentication. | |
151 | .Cm AuthorizedKeysFile | |
152 | may contain tokens of the form %T which are substituted during connection | |
b74c3b8c | 153 | setup. |
3cbc677d | 154 | The following tokens are defined: %% is replaced by a literal '%', |
b74c3b8c | 155 | %h is replaced by the home directory of the user being authenticated, and |
588df31a | 156 | %u is replaced by the username of that user. |
157 | After expansion, | |
158 | .Cm AuthorizedKeysFile | |
159 | is taken to be an absolute path or one relative to the user's home | |
160 | directory. | |
161 | The default is | |
162 | .Dq .ssh/authorized_keys . | |
163 | .It Cm Banner | |
164 | In some jurisdictions, sending a warning message before authentication | |
165 | may be relevant for getting legal protection. | |
166 | The contents of the specified file are sent to the remote user before | |
167 | authentication is allowed. | |
168 | This option is only available for protocol version 2. | |
169 | By default, no banner is displayed. | |
588df31a | 170 | .It Cm ChallengeResponseAuthentication |
340a4caf | 171 | Specifies whether challenge-response authentication is allowed. |
588df31a | 172 | All authentication styles from |
173 | .Xr login.conf 5 | |
174 | are supported. | |
175 | The default is | |
176 | .Dq yes . | |
177 | .It Cm Ciphers | |
178 | Specifies the ciphers allowed for protocol version 2. | |
179 | Multiple ciphers must be comma-separated. | |
3b9baa7b | 180 | The supported ciphers are |
181 | .Dq 3des-cbc , | |
182 | .Dq aes128-cbc , | |
183 | .Dq aes192-cbc , | |
184 | .Dq aes256-cbc , | |
185 | .Dq aes128-ctr , | |
186 | .Dq aes192-ctr , | |
187 | .Dq aes256-ctr , | |
74a66cc8 | 188 | .Dq arcfour128 , |
189 | .Dq arcfour256 , | |
3b9baa7b | 190 | .Dq arcfour , |
191 | .Dq blowfish-cbc , | |
192 | and | |
193 | .Dq cast128-cbc . | |
b74c3b8c | 194 | The default is: |
195 | .Bd -literal -offset 3n | |
196 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | |
197 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | |
198 | aes192-ctr,aes256-ctr | |
588df31a | 199 | .Ed |
588df31a | 200 | .It Cm ClientAliveCountMax |
79e46360 | 201 | Sets the number of client alive messages (see below) which may be |
588df31a | 202 | sent without |
b74c3b8c | 203 | .Xr sshd 8 |
3cbc677d | 204 | receiving any messages back from the client. |
205 | If this threshold is reached while client alive messages are being sent, | |
b74c3b8c | 206 | sshd will disconnect the client, terminating the session. |
3cbc677d | 207 | It is important to note that the use of client alive messages is very |
208 | different from | |
fd573618 | 209 | .Cm TCPKeepAlive |
3cbc677d | 210 | (below). |
211 | The client alive messages are sent through the encrypted channel | |
212 | and therefore will not be spoofable. | |
213 | The TCP keepalive option enabled by | |
fd573618 | 214 | .Cm TCPKeepAlive |
3cbc677d | 215 | is spoofable. |
216 | The client alive mechanism is valuable when the client or | |
588df31a | 217 | server depend on knowing when a connection has become inactive. |
218 | .Pp | |
3cbc677d | 219 | The default value is 3. |
220 | If | |
588df31a | 221 | .Cm ClientAliveInterval |
79e46360 | 222 | (see below) is set to 15, and |
588df31a | 223 | .Cm ClientAliveCountMax |
b74c3b8c | 224 | is left at the default, unresponsive SSH clients |
588df31a | 225 | will be disconnected after approximately 45 seconds. |
2d762582 | 226 | This option applies to protocol version 2 only. |
2264526c | 227 | .It Cm ClientAliveInterval |
228 | Sets a timeout interval in seconds after which if no data has been received | |
229 | from the client, | |
b74c3b8c | 230 | .Xr sshd 8 |
2264526c | 231 | will send a message through the encrypted |
232 | channel to request a response from the client. | |
233 | The default | |
234 | is 0, indicating that these messages will not be sent to the client. | |
235 | This option applies to protocol version 2 only. | |
588df31a | 236 | .It Cm Compression |
07200973 | 237 | Specifies whether compression is allowed, or delayed until |
238 | the user has authenticated successfully. | |
588df31a | 239 | The argument must be |
07200973 | 240 | .Dq yes , |
241 | .Dq delayed , | |
588df31a | 242 | or |
243 | .Dq no . | |
244 | The default is | |
07200973 | 245 | .Dq delayed . |
588df31a | 246 | .It Cm DenyGroups |
247 | This keyword can be followed by a list of group name patterns, separated | |
248 | by spaces. | |
249 | Login is disallowed for users whose primary group or supplementary | |
250 | group list matches one of the patterns. | |
588df31a | 251 | Only group names are valid; a numerical group ID is not recognized. |
252 | By default, login is allowed for all groups. | |
84c1b530 | 253 | The allow/deny directives are processed in the following order: |
254 | .Cm DenyUsers , | |
255 | .Cm AllowUsers , | |
256 | .Cm DenyGroups , | |
257 | and finally | |
258 | .Cm AllowGroups . | |
ac1ec4d8 | 259 | .Pp |
260 | See | |
261 | .Sx PATTERNS | |
262 | in | |
263 | .Xr ssh_config 5 | |
264 | for more information on patterns. | |
588df31a | 265 | .It Cm DenyUsers |
266 | This keyword can be followed by a list of user name patterns, separated | |
267 | by spaces. | |
268 | Login is disallowed for user names that match one of the patterns. | |
588df31a | 269 | Only user names are valid; a numerical user ID is not recognized. |
270 | By default, login is allowed for all users. | |
271 | If the pattern takes the form USER@HOST then USER and HOST | |
272 | are separately checked, restricting logins to particular | |
273 | users from particular hosts. | |
84c1b530 | 274 | The allow/deny directives are processed in the following order: |
275 | .Cm DenyUsers , | |
276 | .Cm AllowUsers , | |
277 | .Cm DenyGroups , | |
278 | and finally | |
279 | .Cm AllowGroups . | |
ac1ec4d8 | 280 | .Pp |
281 | See | |
282 | .Sx PATTERNS | |
283 | in | |
284 | .Xr ssh_config 5 | |
285 | for more information on patterns. | |
e7259e8d | 286 | .It Cm ForceCommand |
287 | Forces the execution of the command specified by | |
288 | .Cm ForceCommand , | |
289 | ignoring any command supplied by the client. | |
290 | The command is invoked by using the user's login shell with the -c option. | |
291 | This applies to shell, command, or subsystem execution. | |
292 | It is most useful inside a | |
293 | .Cm Match | |
294 | block. | |
295 | The command originally supplied by the client is available in the | |
296 | .Ev SSH_ORIGINAL_COMMAND | |
297 | environment variable. | |
588df31a | 298 | .It Cm GatewayPorts |
299 | Specifies whether remote hosts are allowed to connect to ports | |
300 | forwarded for the client. | |
301 | By default, | |
b74c3b8c | 302 | .Xr sshd 8 |
a4e5acef | 303 | binds remote port forwardings to the loopback address. |
304 | This prevents other remote hosts from connecting to forwarded ports. | |
588df31a | 305 | .Cm GatewayPorts |
b74c3b8c | 306 | can be used to specify that sshd |
3867aa0a | 307 | should allow remote port forwardings to bind to non-loopback addresses, thus |
308 | allowing other hosts to connect. | |
309 | The argument may be | |
310 | .Dq no | |
311 | to force remote port forwardings to be available to the local host only, | |
588df31a | 312 | .Dq yes |
3867aa0a | 313 | to force remote port forwardings to bind to the wildcard address, or |
314 | .Dq clientspecified | |
315 | to allow the client to select the address to which the forwarding is bound. | |
588df31a | 316 | The default is |
317 | .Dq no . | |
7364bd04 | 318 | .It Cm GSSAPIAuthentication |
105b07db | 319 | Specifies whether user authentication based on GSSAPI is allowed. |
aff51935 | 320 | The default is |
7364bd04 | 321 | .Dq no . |
322 | Note that this option applies to protocol version 2 only. | |
323 | .It Cm GSSAPICleanupCredentials | |
324 | Specifies whether to automatically destroy the user's credentials cache | |
325 | on logout. | |
326 | The default is | |
327 | .Dq yes . | |
328 | Note that this option applies to protocol version 2 only. | |
588df31a | 329 | .It Cm HostbasedAuthentication |
330 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
331 | with successful public key client host authentication is allowed | |
340a4caf | 332 | (host-based authentication). |
588df31a | 333 | This option is similar to |
334 | .Cm RhostsRSAAuthentication | |
335 | and applies to protocol version 2 only. | |
336 | The default is | |
337 | .Dq no . | |
338 | .It Cm HostKey | |
339 | Specifies a file containing a private host key | |
340 | used by SSH. | |
341 | The default is | |
342 | .Pa /etc/ssh/ssh_host_key | |
343 | for protocol version 1, and | |
344 | .Pa /etc/ssh/ssh_host_rsa_key | |
345 | and | |
346 | .Pa /etc/ssh/ssh_host_dsa_key | |
347 | for protocol version 2. | |
348 | Note that | |
b74c3b8c | 349 | .Xr sshd 8 |
588df31a | 350 | will refuse to use a file if it is group/world-accessible. |
351 | It is possible to have multiple host key files. | |
352 | .Dq rsa1 | |
353 | keys are used for version 1 and | |
354 | .Dq dsa | |
355 | or | |
356 | .Dq rsa | |
357 | are used for version 2 of the SSH protocol. | |
358 | .It Cm IgnoreRhosts | |
359 | Specifies that | |
360 | .Pa .rhosts | |
361 | and | |
362 | .Pa .shosts | |
363 | files will not be used in | |
588df31a | 364 | .Cm RhostsRSAAuthentication |
365 | or | |
366 | .Cm HostbasedAuthentication . | |
367 | .Pp | |
368 | .Pa /etc/hosts.equiv | |
369 | and | |
370 | .Pa /etc/shosts.equiv | |
371 | are still used. | |
372 | The default is | |
373 | .Dq yes . | |
374 | .It Cm IgnoreUserKnownHosts | |
375 | Specifies whether | |
b74c3b8c | 376 | .Xr sshd 8 |
588df31a | 377 | should ignore the user's |
140e3e97 | 378 | .Pa ~/.ssh/known_hosts |
588df31a | 379 | during |
380 | .Cm RhostsRSAAuthentication | |
381 | or | |
382 | .Cm HostbasedAuthentication . | |
383 | The default is | |
384 | .Dq no . | |
588df31a | 385 | .It Cm KerberosAuthentication |
8f73f7bb | 386 | Specifies whether the password provided by the user for |
588df31a | 387 | .Cm PasswordAuthentication |
8f73f7bb | 388 | will be validated through the Kerberos KDC. |
588df31a | 389 | To use this option, the server needs a |
390 | Kerberos servtab which allows the verification of the KDC's identity. | |
b74c3b8c | 391 | The default is |
588df31a | 392 | .Dq no . |
24f37810 | 393 | .It Cm KerberosGetAFSToken |
0d3d1077 | 394 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire |
24f37810 | 395 | an AFS token before accessing the user's home directory. |
b74c3b8c | 396 | The default is |
24f37810 | 397 | .Dq no . |
588df31a | 398 | .It Cm KerberosOrLocalPasswd |
b74c3b8c | 399 | If password authentication through Kerberos fails then |
588df31a | 400 | the password will be validated via any additional local mechanism |
401 | such as | |
402 | .Pa /etc/passwd . | |
b74c3b8c | 403 | The default is |
588df31a | 404 | .Dq yes . |
588df31a | 405 | .It Cm KerberosTicketCleanup |
406 | Specifies whether to automatically destroy the user's ticket cache | |
407 | file on logout. | |
b74c3b8c | 408 | The default is |
588df31a | 409 | .Dq yes . |
410 | .It Cm KeyRegenerationInterval | |
411 | In protocol version 1, the ephemeral server key is automatically regenerated | |
412 | after this many seconds (if it has been used). | |
413 | The purpose of regeneration is to prevent | |
414 | decrypting captured sessions by later breaking into the machine and | |
415 | stealing the keys. | |
416 | The key is never stored anywhere. | |
417 | If the value is 0, the key is never regenerated. | |
418 | The default is 3600 (seconds). | |
419 | .It Cm ListenAddress | |
420 | Specifies the local addresses | |
b74c3b8c | 421 | .Xr sshd 8 |
588df31a | 422 | should listen on. |
423 | The following forms may be used: | |
424 | .Pp | |
425 | .Bl -item -offset indent -compact | |
426 | .It | |
427 | .Cm ListenAddress | |
428 | .Sm off | |
429 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
430 | .Sm on | |
431 | .It | |
432 | .Cm ListenAddress | |
433 | .Sm off | |
434 | .Ar host No | Ar IPv4_addr No : Ar port | |
435 | .Sm on | |
436 | .It | |
437 | .Cm ListenAddress | |
438 | .Sm off | |
439 | .Oo | |
440 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
441 | .Sm on | |
442 | .El | |
443 | .Pp | |
444 | If | |
445 | .Ar port | |
446 | is not specified, | |
b74c3b8c | 447 | sshd will listen on the address and all prior |
588df31a | 448 | .Cm Port |
3cbc677d | 449 | options specified. |
450 | The default is to listen on all local addresses. | |
a4e5acef | 451 | Multiple |
588df31a | 452 | .Cm ListenAddress |
3cbc677d | 453 | options are permitted. |
454 | Additionally, any | |
588df31a | 455 | .Cm Port |
b74c3b8c | 456 | options must precede this option for non-port qualified addresses. |
588df31a | 457 | .It Cm LoginGraceTime |
458 | The server disconnects after this time if the user has not | |
459 | successfully logged in. | |
460 | If the value is 0, there is no time limit. | |
3445ca02 | 461 | The default is 120 seconds. |
588df31a | 462 | .It Cm LogLevel |
463 | Gives the verbosity level that is used when logging messages from | |
32cfd177 | 464 | .Xr sshd 8 . |
588df31a | 465 | The possible values are: |
b74c3b8c | 466 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. |
a4e5acef | 467 | The default is INFO. |
468 | DEBUG and DEBUG1 are equivalent. | |
469 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
470 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
588df31a | 471 | .It Cm MACs |
472 | Specifies the available MAC (message authentication code) algorithms. | |
473 | The MAC algorithm is used in protocol version 2 | |
474 | for data integrity protection. | |
475 | Multiple algorithms must be comma-separated. | |
b74c3b8c | 476 | The default is: |
588df31a | 477 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
d231781a | 478 | .It Cm Match |
14e980ef | 479 | Introduces a conditional block. |
4895f836 | 480 | If all of the criteria on the |
d231781a | 481 | .Cm Match |
4895f836 | 482 | line are satisfied, the keywords on the following lines override those |
483 | set in the global section of the config file, until either another | |
d231781a | 484 | .Cm Match |
4895f836 | 485 | line or the end of the file. |
14e980ef | 486 | The arguments to |
d231781a | 487 | .Cm Match |
4895f836 | 488 | are one or more criteria-pattern pairs. |
d231781a | 489 | The available criteria are |
490 | .Cm User , | |
491 | .Cm Host , | |
492 | and | |
493 | .Cm Address . | |
494 | Only a subset of keywords may be used on the lines following a | |
495 | .Cm Match | |
496 | keyword. | |
497 | Available keywords are | |
2fefbadf | 498 | .Cm AllowTcpForwarding , |
e7259e8d | 499 | .Cm ForceCommand , |
2fefbadf | 500 | .Cm GatewayPorts , |
691712e0 | 501 | .Cm PermitOpen , |
502 | .Cm X11DisplayOffset , | |
503 | .Cm X11Forwarding , | |
d231781a | 504 | and |
691712e0 | 505 | .Cm X11UseLocalHost . |
af4bd935 | 506 | .It Cm MaxAuthTries |
507 | Specifies the maximum number of authentication attempts permitted per | |
8fca654b | 508 | connection. |
509 | Once the number of failures reaches half this value, | |
510 | additional failures are logged. | |
511 | The default is 6. | |
588df31a | 512 | .It Cm MaxStartups |
513 | Specifies the maximum number of concurrent unauthenticated connections to the | |
b74c3b8c | 514 | SSH daemon. |
588df31a | 515 | Additional connections will be dropped until authentication succeeds or the |
516 | .Cm LoginGraceTime | |
517 | expires for a connection. | |
518 | The default is 10. | |
519 | .Pp | |
520 | Alternatively, random early drop can be enabled by specifying | |
521 | the three colon separated values | |
522 | .Dq start:rate:full | |
f09ffbdb | 523 | (e.g. "10:30:60"). |
32cfd177 | 524 | .Xr sshd 8 |
588df31a | 525 | will refuse connection attempts with a probability of |
526 | .Dq rate/100 | |
527 | (30%) | |
528 | if there are currently | |
529 | .Dq start | |
530 | (10) | |
531 | unauthenticated connections. | |
532 | The probability increases linearly and all connection attempts | |
533 | are refused if the number of unauthenticated connections reaches | |
534 | .Dq full | |
535 | (60). | |
536 | .It Cm PasswordAuthentication | |
537 | Specifies whether password authentication is allowed. | |
538 | The default is | |
539 | .Dq yes . | |
540 | .It Cm PermitEmptyPasswords | |
541 | When password authentication is allowed, it specifies whether the | |
542 | server allows login to accounts with empty password strings. | |
543 | The default is | |
544 | .Dq no . | |
2fefbadf | 545 | .It Cm PermitOpen |
546 | Specifies the destinations to which TCP port forwarding is permitted. | |
547 | The forwarding specification must be one of the following forms: | |
548 | .Pp | |
549 | .Bl -item -offset indent -compact | |
550 | .It | |
551 | .Cm PermitOpen | |
552 | .Sm off | |
553 | .Ar host : port | |
554 | .Sm on | |
555 | .It | |
556 | .Cm PermitOpen | |
557 | .Sm off | |
558 | .Ar IPv4_addr : port | |
559 | .Sm on | |
560 | .It | |
561 | .Cm PermitOpen | |
562 | .Sm off | |
563 | .Ar \&[ IPv6_addr \&] : port | |
564 | .Sm on | |
565 | .El | |
566 | .Pp | |
ea46e550 | 567 | Multiple forwards may be specified by separating them with whitespace. |
2fefbadf | 568 | An argument of |
569 | .Dq any | |
570 | can be used to remove all restrictions and permit any forwarding requests. | |
f22506ff | 571 | By default all port forwarding requests are permitted. |
588df31a | 572 | .It Cm PermitRootLogin |
667e4135 | 573 | Specifies whether root can log in using |
588df31a | 574 | .Xr ssh 1 . |
575 | The argument must be | |
576 | .Dq yes , | |
577 | .Dq without-password , | |
b74c3b8c | 578 | .Dq forced-commands-only , |
588df31a | 579 | or |
580 | .Dq no . | |
581 | The default is | |
582 | .Dq yes . | |
583 | .Pp | |
584 | If this option is set to | |
b74c3b8c | 585 | .Dq without-password , |
8a4c4ee4 | 586 | password authentication is disabled for root. |
588df31a | 587 | .Pp |
588 | If this option is set to | |
b74c3b8c | 589 | .Dq forced-commands-only , |
588df31a | 590 | root login with public key authentication will be allowed, |
591 | but only if the | |
592 | .Ar command | |
593 | option has been specified | |
594 | (which may be useful for taking remote backups even if root login is | |
3cbc677d | 595 | normally not allowed). |
596 | All other authentication methods are disabled for root. | |
588df31a | 597 | .Pp |
598 | If this option is set to | |
b74c3b8c | 599 | .Dq no , |
667e4135 | 600 | root is not allowed to log in. |
d20f3c9e | 601 | .It Cm PermitTunnel |
602 | Specifies whether | |
603 | .Xr tun 4 | |
604 | device forwarding is allowed. | |
a4f24bf8 | 605 | The argument must be |
606 | .Dq yes , | |
06fa4ac1 | 607 | .Dq point-to-point |
608 | (layer 3), | |
609 | .Dq ethernet | |
610 | (layer 2), or | |
a4f24bf8 | 611 | .Dq no . |
06fa4ac1 | 612 | Specifying |
613 | .Dq yes | |
614 | permits both | |
615 | .Dq point-to-point | |
616 | and | |
617 | .Dq ethernet . | |
d20f3c9e | 618 | The default is |
619 | .Dq no . | |
f00bab84 | 620 | .It Cm PermitUserEnvironment |
621 | Specifies whether | |
622 | .Pa ~/.ssh/environment | |
35453849 | 623 | and |
f00bab84 | 624 | .Cm environment= |
625 | options in | |
626 | .Pa ~/.ssh/authorized_keys | |
35453849 | 627 | are processed by |
b74c3b8c | 628 | .Xr sshd 8 . |
f00bab84 | 629 | The default is |
630 | .Dq no . | |
35453849 | 631 | Enabling environment processing may enable users to bypass access |
632 | restrictions in some configurations using mechanisms such as | |
633 | .Ev LD_PRELOAD . | |
588df31a | 634 | .It Cm PidFile |
baa08b92 | 635 | Specifies the file that contains the process ID of the |
32cfd177 | 636 | SSH daemon. |
588df31a | 637 | The default is |
638 | .Pa /var/run/sshd.pid . | |
639 | .It Cm Port | |
640 | Specifies the port number that | |
b74c3b8c | 641 | .Xr sshd 8 |
588df31a | 642 | listens on. |
643 | The default is 22. | |
644 | Multiple options of this type are permitted. | |
645 | See also | |
646 | .Cm ListenAddress . | |
647 | .It Cm PrintLastLog | |
648 | Specifies whether | |
b74c3b8c | 649 | .Xr sshd 8 |
329a8666 | 650 | should print the date and time of the last user login when a user logs |
651 | in interactively. | |
588df31a | 652 | The default is |
653 | .Dq yes . | |
654 | .It Cm PrintMotd | |
655 | Specifies whether | |
b74c3b8c | 656 | .Xr sshd 8 |
588df31a | 657 | should print |
658 | .Pa /etc/motd | |
659 | when a user logs in interactively. | |
660 | (On some systems it is also printed by the shell, | |
661 | .Pa /etc/profile , | |
662 | or equivalent.) | |
663 | The default is | |
664 | .Dq yes . | |
665 | .It Cm Protocol | |
666 | Specifies the protocol versions | |
b74c3b8c | 667 | .Xr sshd 8 |
94ad46d1 | 668 | supports. |
588df31a | 669 | The possible values are |
b74c3b8c | 670 | .Sq 1 |
588df31a | 671 | and |
b74c3b8c | 672 | .Sq 2 . |
588df31a | 673 | Multiple versions must be comma-separated. |
674 | The default is | |
675 | .Dq 2,1 . | |
94ad46d1 | 676 | Note that the order of the protocol list does not indicate preference, |
677 | because the client selects among multiple protocol versions offered | |
678 | by the server. | |
679 | Specifying | |
680 | .Dq 2,1 | |
681 | is identical to | |
682 | .Dq 1,2 . | |
588df31a | 683 | .It Cm PubkeyAuthentication |
684 | Specifies whether public key authentication is allowed. | |
685 | The default is | |
686 | .Dq yes . | |
687 | Note that this option applies to protocol version 2 only. | |
588df31a | 688 | .It Cm RhostsRSAAuthentication |
689 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
690 | with successful RSA host authentication is allowed. | |
691 | The default is | |
692 | .Dq no . | |
693 | This option applies to protocol version 1 only. | |
694 | .It Cm RSAAuthentication | |
695 | Specifies whether pure RSA authentication is allowed. | |
696 | The default is | |
697 | .Dq yes . | |
698 | This option applies to protocol version 1 only. | |
699 | .It Cm ServerKeyBits | |
700 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
701 | The minimum value is 512, and the default is 768. | |
702 | .It Cm StrictModes | |
703 | Specifies whether | |
b74c3b8c | 704 | .Xr sshd 8 |
588df31a | 705 | should check file modes and ownership of the |
706 | user's files and home directory before accepting login. | |
707 | This is normally desirable because novices sometimes accidentally leave their | |
708 | directory or files world-writable. | |
709 | The default is | |
710 | .Dq yes . | |
711 | .It Cm Subsystem | |
f09ffbdb | 712 | Configures an external subsystem (e.g. file transfer daemon). |
d66ce1a1 | 713 | Arguments should be a subsystem name and a command (with optional arguments) |
714 | to execute upon subsystem request. | |
588df31a | 715 | The command |
716 | .Xr sftp-server 8 | |
717 | implements the | |
718 | .Dq sftp | |
719 | file transfer subsystem. | |
720 | By default no subsystems are defined. | |
721 | Note that this option applies to protocol version 2 only. | |
722 | .It Cm SyslogFacility | |
723 | Gives the facility code that is used when logging messages from | |
32cfd177 | 724 | .Xr sshd 8 . |
588df31a | 725 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
726 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
727 | The default is AUTH. | |
fd573618 | 728 | .It Cm TCPKeepAlive |
729 | Specifies whether the system should send TCP keepalive messages to the | |
730 | other side. | |
731 | If they are sent, death of the connection or crash of one | |
732 | of the machines will be properly noticed. | |
733 | However, this means that | |
734 | connections will die if the route is down temporarily, and some people | |
735 | find it annoying. | |
736 | On the other hand, if TCP keepalives are not sent, | |
737 | sessions may hang indefinitely on the server, leaving | |
738 | .Dq ghost | |
739 | users and consuming server resources. | |
740 | .Pp | |
741 | The default is | |
742 | .Dq yes | |
743 | (to send TCP keepalive messages), and the server will notice | |
744 | if the network goes down or the client host crashes. | |
745 | This avoids infinitely hanging sessions. | |
746 | .Pp | |
747 | To disable TCP keepalive messages, the value should be set to | |
748 | .Dq no . | |
c5a7d788 | 749 | .It Cm UseDNS |
750 | Specifies whether | |
b74c3b8c | 751 | .Xr sshd 8 |
102c77c2 | 752 | should look up the remote host name and check that |
c5a7d788 | 753 | the resolved host name for the remote IP address maps back to the |
754 | very same IP address. | |
755 | The default is | |
756 | .Dq yes . | |
588df31a | 757 | .It Cm UseLogin |
758 | Specifies whether | |
759 | .Xr login 1 | |
760 | is used for interactive login sessions. | |
761 | The default is | |
762 | .Dq no . | |
763 | Note that | |
764 | .Xr login 1 | |
765 | is never used for remote command execution. | |
766 | Note also, that if this is enabled, | |
767 | .Cm X11Forwarding | |
768 | will be disabled because | |
769 | .Xr login 1 | |
770 | does not know how to handle | |
771 | .Xr xauth 1 | |
a4e5acef | 772 | cookies. |
773 | If | |
588df31a | 774 | .Cm UsePrivilegeSeparation |
775 | is specified, it will be disabled after authentication. | |
72c35df7 | 776 | .It Cm UsePAM |
d5c67850 | 777 | Enables the Pluggable Authentication Module interface. |
778 | If set to | |
779 | .Dq yes | |
780 | this will enable PAM authentication using | |
781 | .Cm ChallengeResponseAuthentication | |
05059810 | 782 | and |
783 | .Cm PasswordAuthentication | |
784 | in addition to PAM account and session module processing for all | |
785 | authentication types. | |
d5c67850 | 786 | .Pp |
787 | Because PAM challenge-response authentication usually serves an equivalent | |
788 | role to password authentication, you should disable either | |
789 | .Cm PasswordAuthentication | |
790 | or | |
791 | .Cm ChallengeResponseAuthentication. | |
792 | .Pp | |
793 | If | |
794 | .Cm UsePAM | |
795 | is enabled, you will not be able to run | |
796 | .Xr sshd 8 | |
797 | as a non-root user. | |
798 | The default is | |
a83a3125 | 799 | .Dq no . |
588df31a | 800 | .It Cm UsePrivilegeSeparation |
801 | Specifies whether | |
b74c3b8c | 802 | .Xr sshd 8 |
588df31a | 803 | separates privileges by creating an unprivileged child process |
a4e5acef | 804 | to deal with incoming network traffic. |
805 | After successful authentication, another process will be created that has | |
806 | the privilege of the authenticated user. | |
807 | The goal of privilege separation is to prevent privilege | |
588df31a | 808 | escalation by containing any corruption within the unprivileged processes. |
809 | The default is | |
810 | .Dq yes . | |
588df31a | 811 | .It Cm X11DisplayOffset |
812 | Specifies the first display number available for | |
b74c3b8c | 813 | .Xr sshd 8 Ns 's |
588df31a | 814 | X11 forwarding. |
b74c3b8c | 815 | This prevents sshd from interfering with real X11 servers. |
588df31a | 816 | The default is 10. |
817 | .It Cm X11Forwarding | |
818 | Specifies whether X11 forwarding is permitted. | |
e6fe1bab | 819 | The argument must be |
820 | .Dq yes | |
821 | or | |
822 | .Dq no . | |
588df31a | 823 | The default is |
824 | .Dq no . | |
e6fe1bab | 825 | .Pp |
826 | When X11 forwarding is enabled, there may be additional exposure to | |
827 | the server and to client displays if the | |
b74c3b8c | 828 | .Xr sshd 8 |
e6fe1bab | 829 | proxy display is configured to listen on the wildcard address (see |
830 | .Cm X11UseLocalhost | |
b74c3b8c | 831 | below), though this is not the default. |
e6fe1bab | 832 | Additionally, the authentication spoofing and authentication data |
833 | verification and substitution occur on the client side. | |
834 | The security risk of using X11 forwarding is that the client's X11 | |
b74c3b8c | 835 | display server may be exposed to attack when the SSH client requests |
e6fe1bab | 836 | forwarding (see the warnings for |
837 | .Cm ForwardX11 | |
838 | in | |
be193d89 | 839 | .Xr ssh_config 5 ) . |
e6fe1bab | 840 | A system administrator may have a stance in which they want to |
841 | protect clients that may expose themselves to attack by unwittingly | |
842 | requesting X11 forwarding, which can warrant a | |
843 | .Dq no | |
844 | setting. | |
845 | .Pp | |
846 | Note that disabling X11 forwarding does not prevent users from | |
847 | forwarding X11 traffic, as users can always install their own forwarders. | |
588df31a | 848 | X11 forwarding is automatically disabled if |
849 | .Cm UseLogin | |
850 | is enabled. | |
851 | .It Cm X11UseLocalhost | |
852 | Specifies whether | |
b74c3b8c | 853 | .Xr sshd 8 |
588df31a | 854 | should bind the X11 forwarding server to the loopback address or to |
a4e5acef | 855 | the wildcard address. |
856 | By default, | |
b74c3b8c | 857 | sshd binds the forwarding server to the loopback address and sets the |
588df31a | 858 | hostname part of the |
859 | .Ev DISPLAY | |
860 | environment variable to | |
861 | .Dq localhost . | |
b3641662 | 862 | This prevents remote hosts from connecting to the proxy display. |
588df31a | 863 | However, some older X11 clients may not function with this |
864 | configuration. | |
865 | .Cm X11UseLocalhost | |
866 | may be set to | |
867 | .Dq no | |
868 | to specify that the forwarding server should be bound to the wildcard | |
869 | address. | |
870 | The argument must be | |
871 | .Dq yes | |
872 | or | |
873 | .Dq no . | |
874 | The default is | |
875 | .Dq yes . | |
876 | .It Cm XAuthLocation | |
57ff5eeb | 877 | Specifies the full pathname of the |
588df31a | 878 | .Xr xauth 1 |
879 | program. | |
880 | The default is | |
881 | .Pa /usr/X11R6/bin/xauth . | |
882 | .El | |
ef1c6497 | 883 | .Sh TIME FORMATS |
32cfd177 | 884 | .Xr sshd 8 |
588df31a | 885 | command-line arguments and configuration file options that specify time |
886 | may be expressed using a sequence of the form: | |
887 | .Sm off | |
36535ee6 | 888 | .Ar time Op Ar qualifier , |
588df31a | 889 | .Sm on |
890 | where | |
891 | .Ar time | |
892 | is a positive integer value and | |
893 | .Ar qualifier | |
894 | is one of the following: | |
895 | .Pp | |
896 | .Bl -tag -width Ds -compact -offset indent | |
874d319b | 897 | .It Aq Cm none |
588df31a | 898 | seconds |
899 | .It Cm s | Cm S | |
900 | seconds | |
901 | .It Cm m | Cm M | |
902 | minutes | |
903 | .It Cm h | Cm H | |
904 | hours | |
905 | .It Cm d | Cm D | |
906 | days | |
907 | .It Cm w | Cm W | |
908 | weeks | |
909 | .El | |
910 | .Pp | |
911 | Each member of the sequence is added together to calculate | |
912 | the total time value. | |
913 | .Pp | |
914 | Time format examples: | |
915 | .Pp | |
916 | .Bl -tag -width Ds -compact -offset indent | |
917 | .It 600 | |
918 | 600 seconds (10 minutes) | |
919 | .It 10m | |
920 | 10 minutes | |
921 | .It 1h30m | |
922 | 1 hour 30 minutes (90 minutes) | |
923 | .El | |
924 | .Sh FILES | |
925 | .Bl -tag -width Ds | |
926 | .It Pa /etc/ssh/sshd_config | |
927 | Contains configuration data for | |
32cfd177 | 928 | .Xr sshd 8 . |
588df31a | 929 | This file should be writable by root only, but it is recommended |
930 | (though not necessary) that it be world-readable. | |
931 | .El | |
be193d89 | 932 | .Sh SEE ALSO |
933 | .Xr sshd 8 | |
588df31a | 934 | .Sh AUTHORS |
935 | OpenSSH is a derivative of the original and free | |
936 | ssh 1.2.12 release by Tatu Ylonen. | |
937 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
938 | Theo de Raadt and Dug Song | |
939 | removed many bugs, re-added newer features and | |
940 | created OpenSSH. | |
941 | Markus Friedl contributed the support for SSH | |
942 | protocol versions 1.5 and 2.0. | |
943 | Niels Provos and Markus Friedl contributed support | |
944 | for privilege separation. |